1. Field of the Invention
The present application relates to a system for extruding multi-layer thermoplastic materials into sheets having selected layer thicknesses and more particularly to modular extrusion manifold systems.
2. Description of Related Art
The invention is concerned with the art of fixed area injection port plastic extruders for the manufacture of thermoplastic sheet or film. It is known in the art that many advantages are achieved by the production of multi-layer film constructions of thermoplastic materials as they exhibit characteristics which are a composite of all the individual layer characteristics. For instance, the inner layer may be of a material with high permeability resistance to chemicals, the next layer may be a pigmented layer followed by a layer which is very tough and a final layer may have an ultraviolet light inhibiter in it to protect the underlying structures from ultraviolet light. These layers are often melt laminated in a co-extrusion process which results in a very stable structure with extremely high delamination resistance. Where several products may be manufactured by a factory, a simple and economical means to vary the number of laminae and the relative thicknesses of each is required.
Fixed manifold dies are well known in the art, for instance the invention of U.S. Pat. No. 4,226,822 which teaches a five-layer film extrusion system. Extruders of fixed orifice size are often employed where, as in that invention's application, a very high degree of laminal thickness control is required. These fixed units become increasingly expensive to build because of their injection chamber configurations' complexity. Consequently, the configurations are generally application specific; relating to a specific combination of laminae and melt materials and are not readily adaptable to extrude sheets with different thermoplastic materials or diverse lamina thicknesses. Furthermore, as extrudate rheology or injection pressure vary during the period of a manufacturing run, it can be difficult to compensate for factors such as viscosity changes when using a fixed manifold system which may result in extrudate laminal thickness inconsistencies.
An example of a variable orifice size die is taught in U.S. Pat. Nos. 4,152,387 and 4,533,308. Dies of these configurations utilize a vane system which directs flow and adjusts lamina thickness, within limits, without requiring a die exchange. Similarly, the system provides for adjustment to accommodate diverse melt rheologies. The inherent disadvantage of such systems lays in the need to experimentally determine the flow distribution settings after each changeover which results in a loss of production capacity. Production capacity is important because high production capacity is the primary advantage of extrusion systems over other thermoplastic production techniques.
Extrudate laminal uniformity is also critical and those factors related to melt flow characteristics which result in diverse flow rates among the passages can cause a curtaining effect, well known in the art, whereby waves occur in the finished sheet due to variations in the passage flow rates. Another effect which has been observed is that there exists a pressure gradient across a melt flow such that the pressure at the center of the flow is greater than that at each extreme edge of the flow. This causes increased melt deposition in the center of the sheet being extruded and a corresponding thinning at the sheet edges which results in a non-uniform extrudate.
The curtaining effect is a result of the convergence of molten thermoplastic streams at diverse flow rates. Many attempts have been made in the art to come to terms with the curtaining effect phenomenon. In configurations of the invention principles of U.S. Pat. Nos. 4,533,308 and 4,152,387, compensation is accomplished by shutting down the extruder and changing the metering vane to one of an appropriate configuration with resultant loss in production time as new settings are experimentally determined.
Conventional machines of this type have also the disadvantage that the vane configurations are inherently weaker than the more rigid structures of fixed manifold systems; such strength being necessary to withstand variations in melt passage pressures which can cause the vane to deflect. Furthermore, the long manifold channel lengths limit the flow rate of the melts thereby placing an inherent limitation on the attainable production capacities in systems using dies of their construction.